The present invention relates to acoustically determining deviations of a projectile from a minimal distance between a projectile and the target it passes under exclusion of transit time errors, particularly for application in movable training targets, under utilization of a suitable microphone system, cooperating with evaluating devices.
Generally speaking, methods for acoustically determining minimal distance deviations of a projectile from a resting training target or from a target moving with subsonic velocity, are based on the following consideration. The projectile is assumed to propagate with supersonic speed and produces a conical shockwave (Mach cone). These shockwaves are ascertained under utilization of at least one, usually several microphones. There is a relationship between the distance of the microphones from the shockwave generating point in any instant being a point on the path of the projectile, and the shockwave amplitude and/or the shockwave duration. These relationships are known. Moreover whenever the target is not moving, than one can derive from these relationships the shortest distance between the projectile represented by the point of shockwave generation, and that target.
It is also known, however, that in case of a moving target the direct measurement is apt to include errors so that, depending upon the various vectors describing the velocity of the projectile, the speed of the target, and the speed of sound, will only rarely yield a correct final result.
In order to avoid these errors one has to consider the spacial as well as the temporal history of the passage of the projectile past the target. For such a passage, one can, owing to the brevity of the process, approximate the target path, as well as the projectile trajectory to be straight lines, and the instantaneous velocity can be regarded as constant for such a short duration. However, any meaningful calculations in this regard are possible only if, in fact, one can relate the projectile path and trajectory to the actual propagation path of the target. For this then, two possibilities are known.
German printed patent application No. 31 22 644 describes a method of correcting information derived in relationship to a flying training target based on a geometry which considers the location of the projectile launching equipment and the target location. Here then it is required that the course, i.e. the path of the training target, is maintained very accurately and is, correspondingly, very accurately predetermined. The same is true as far as the altitude and the speed are concerned, and one needs exact distances from the projectile launch site and also the speed of the projectile; any changes of the speed have to be very accurately known. The microphones, moreover, have to be located in the center of the target and the entire arrangement requires an acoustic spherical characteristic.
Another possibility is described in European Pat. No. 3,095. Herein a three-dimensional arrangement is suggested which includes a system of microphones being comprised of at least four microphones and there is a supplemental system, so that all together five microphones are needed. One needs also a very accurately known target related geometry which renders the system independent from the altitude and the propagation course and path of the training target. The microphones, in this case, can be situated outside of the target center.